Motor vehicles having a gasoline-driven internal combustion engine are equipped, these days, with on-board devices which capture the gasoline vapors accumulating during the operation of the stopping phase of the motor vehicle in an activated charcoal filter, so that they do not get out into the environment. The drive system of such a motor vehicle is shown schematically in FIG. 1. Gasoline is stored in fuel tank 11 having a filling orifice 111. Degassing gasoline vapors from fuel tank 11 get into an active charcoal filter 121 of a fuel vapor recirculation system 12. Active charcoal filter 121 has a fresh air exit, so that fuel tank 11 is always pressureless. In order to avoid that active charcoal filter 121 “runs over”, it is regenerated or desorbed in the operating phases of the internal combustion engine. For this, a dosing valve, or rather, a fuel tank vent valve 122 is opened. Fresh air flows through active charcoal filter 121 and guides the gasoline vapors adsorbed in it along, which are supplied downstream of a throttle valve 131 to an intake manifold 13. Through intake manifold 13, they are finally supplied to the combustion in internal combustion engine 14. The assumption is that in intake manifold 13 a certain underpressure prevails, that is, throttle valve 131 is not open to such a great extent. In full-load operation of internal combustion engine 14, for instance in the case of uphill travel, the intake-manifold pressure approaches the environmental pressure, and the pressure difference at tank ventilating valve 122 drops off With that, the desorbed quantity of gasoline vapors through tank ventilation valve 122 also drops off.
In the same way, a “nervous” driving style, which is characterized by high dynamics of the gas pedal, and with that, also of throttle valve 131, is able to lead to a lower regeneration quantity than a “quiet” driving style, which is typically recommended for a fuel-saving driving manner.
The regenerating operation takes place in a so-called time slice control, in which a regenerating phase is cyclically interrupted by a so-called base adaptation phase. The reason for this is that, in the base adaptation phase, basically mixture errors, such as a slow drifting of the fuel injectors is able to be identified, without being superimposed by the short-term, and frequently greatly fluctuating effect of the tank ventilation. The cyclically occurring base adaptation does, however, lead to the regenerating air quantity being restricted.
A further restriction of the generating air quantity takes place because tank ventilating valve 122 is only able to be opened to the extent that the gasoline vapor mass does not exceed the requirement of internal combustion engine 14 for fuel. Otherwise, internal combustion engine 14 would become overrich and would finally shut down. In practice, in the operating strategy of internal combustion engine 14, a great distance from the overriching boundary is maintained, for which the fuel supply, because of tank ventilation valve 122, usually makes no more than 30 to 40% difference in the fuel requirement of internal combustion engine 14.
The manner of functioning of tank ventilation valve 122 is monitored by various lawmakers in the course of certifying motor vehicles. For this, active charcoal filter 121 is removed before travel begins, and loaded with a test gas, so that it is saturated. Thereafter, activated charcoal filter 121 is installed in the vehicle, and, during travel operation, sufficient regeneration has to take place so that enough filtering capacity is available to take up the gasoline vapors accumulating from fuel tank 11 during travel. All motor vehicles which at least satisfy exhaust standard EU2, today have a tank ventilation valve 122, that is, for example, all newly admitted motor vehicles in the USA, in the European Union, in South Korea and in Japan.
The torque generated by internal combustion engine 14 is passed on to a transmission 15. Hybrid vehicles which, besides fuel tank 11 and internal combustion engine 14 also have a battery 16, which supplies an electric motor 17 with power, have operating phases in which electric motor 17 is running and is passing on, via transmission 15, its torque to a drive axle 18 and wheels 181, 182 fastened to it while internal combustion engine 14 is shut down. The shifting over between phases of the internal combustion engine operation and the electric motor operation takes place by a control unit 19. When internal combustion engine 14 is shut down and electric motor 17 is switched on, there can be no regeneration of fuel vapor recirculation system 12 taking place, although new gasoline vapor is constantly degassing from fuel tank 11 and is being absorbed in activated charcoal filter 121. The low purge air quantity of activated charcoal filter 121 leads to the fact that, for such hybrid vehicles, a high technical effort has to be made to pass the certification. Thus, it is known, for example, in hybrid vehicles that fuel tank 11 should be developed as a pressure tank, which holds fuel vapors at overpressure, so that they cannot flow into activated charcoal filter 121. In addition, in such hybrid vehicles, in which internal combustion engine 14 runs only rarely, there is the danger that activated charcoal filter 121 is saturated during operation and “runs over”. This leads to the motor vehicle smelling of gasoline vapors, which leads to a bad vehicle image.